1. Field of the Invention
The present invention relates to an a x-ray device, and more particularly, to an x-ray apparatus such as an x-ray exposure apparatus, an x-ray microscope, or an x-ray analysis device.
The present invention also relates to an x-ray generating apparatus for use as an x-ray source in an x-ray apparatus such as an x-ray exposure apparatus, an x-ray microscope, or an x-ray analysis device.
2. Description of the Related Art
An apparatus, which uses a reduction and projection type exposure by light technique, is currently widely used for the manufacture of semiconductor integrated circuits that have fine semiconductor integrated circuit patterns. Examples of such an apparatus include an x-ray exposure apparatus, an x-ray microscope, and an x-ray analysis device. This apparatus also includes an x-ray generating apparatus as an x-ray source, and a plurality of optical elements.
The wavelength of the light used in such an apparatus has become progressively shorter as patterns have become finer. As a result, attempts have been made to use soft x-rays for the formation of fine patterns, which are, in principle, impossible to achieve using visible or ultraviolet light. However, in the soft x-ray region that includes wavelengths of 1 to 100 nm, all substances show a strong absorption. Accordingly, transmission type optical elements that utilize refraction in the visible light region are not used. Instead, extremely thin-film filters or reflective mirrors, which have multi-layer films formed on them, are used as optical elements. In particular, multi-layer film mirrors have been manufactured in which a direct-incidence reflectivity of approximately 70% is obtained in the vicinity of wavelengths of 11 nm and 13 nm, and the reduction and projection type exposure apparatus with reflective optical systems using such multi-layer films has been proposed.
In cases where thin-film filters or multi-layer films are used, contaminants adhering to the surfaces of such elements are a serious problem. Furthermore, all substances show a strong absorption in the soft x-ray region. Moreover, if a device contains sliding parts such as sample stages, fine particles are generated, and these particles might adhere to the surfaces of soft x-ray optical elements. Soft x-rays are also absorbed by air, and therefore, the light path must be evacuated to a vacuum state. However, there is a slight back flow of oil from ordinary evacuation systems. Under such conditions, when soft x-ray irradiation is performed, carbon contaminants adhere to the surfaces of soft x-ray optical elements. In addition, in an x-ray exposure apparatus, wafers coated with light sensitive polymer known as resists are conveyed into the vacuum vessel during exposure, and such resists generate small amounts of gases that may also cause carbon contamination. These contaminants cause a drop in soft x-ray transmissivity and reflectivity.
Moreover, the x-ray generating apparatus that is used as the x-ray source in an x-ray apparatus also generates particles that adhere to optical elements of the apparatus. An example of an x-ray source is a laser plasma x-ray source or an LPX. In all LPX, pulsed laser light (one example of an exciting energy beam) is focused and directed onto a target material placed in a vacuum vessel under reduced pressure. The target material is rapidly converted into plasma, and x-rays with an extremely high brightness are emitted from this plasma. In particular, such x-ray apparatus are compact, but have a brightness comparable to that of undulators.
Furthermore, an LPX differs from synchrotron radiation light since a high degree of vacuum, such as approximately 10xe2x88x929 torr, is not required. It is only necessary for the degree of vacuum to be high enough to prevent gas discharge due to residual gas before the laser light reaches the surface of the target; or to prevent strong absorption of the x-rays generated from the plasma before these x-rays reach the object of irradiation. In particular, a pressure from several tens of torr to 0.1 torr is sufficient. Accordingly, an inexpensive vacuum evacuation device such as a rotary pump is sufficient, and can easily be used. Thus, such an LPX has attracted attention in recent years for use as an x-ray source in x-ray exposure apparatus, x-ray microscopes, or x-ray analysis devices.
In an LPX, particles such as high-velocity ions and electrons, and particles of materials from the target material such as gasified materials, ionized materials, and material fragments are emitted from the plasma along with the x-rays. These particles scatter throughout the interior of the vacuum vessel. In an x-ray apparatus, which uses an LPX as an x-ray source, such scattered particles adhere to the surfaces of soft x-ray optical elements and cause problems, such as reduction in reflectivity and transmissivity. Accordingly, it is necessary to reduce the quantities of such particles that are generated, and to remove scattered particles that adhere to the optical elements.
Various methods are used to reduce and remove contamination. A wet cleaning method is the most commonly used method to clean the surfaces of contaminated substances. This method consists of treatment by a chemical agent and rinsing by pure water. This cleaning method is very widely used in semiconductor integrated circuit manufacturing processes, and is capable of removing fine adhering substances if the purity of the chemical solution and pure water is increased. However, the wet cleaning method has various problems including consumption of large quantities of cleaning solution and pure water. As a result, in recent years, other cleaning methods, such as the blowing of fine water droplets onto the objects being cleaned at super-high velocities and irradiation with pulsed light, have been proposed.
The latter method of irradiation with pulsed light is effective if a light of a wavelength that can be strongly absorbed by the adhering matter or the substrate is used. When the pulsed light abruptly heats a slight surface portion of the substance, then this surface portion expands. However, when heating occurs in a short time by pulsed light, the displacement acceleration is extremely large, and as a result, the fine particles adhering to the surface can be removed. Furthermore, even in cases where adhering matter is formed in a thin-film configuration on the surface, instantaneous peeling occurs during irradiation with pulsed light. In such cases, the adhering matter is removed, as in the case of fine particles, due to a result of differences in the physical characteristics such as the thermal expansion rate of the underlying material.
Moreover, if the adhering matter is organic matter consisting chiefly of carbon, the removal can be achieved by irradiation with ultraviolet light in an appropriate oxygen atmosphere. In particular, the bonds between carbon atoms in the adhering matter are cleaved by the ultraviolet light. As a result, the carbon bonds with oxygen and is removed in the form of carbon dioxide.
The possibility of some type of substance adhering to the surfaces of soft x-ray optical elements placed in a vacuum cannot be completely eliminated. However, the soft x-ray optical systems are extremely sensitive to surface contamination, and it is necessary to remove substances adhering to such soft x-ray optical elements. However, cleaning in a vacuum using wet cleaning methods, which are currently the most common methods in use, is undesirable because in order to perform cleaning in such cases, it is necessary to remove the soft x-ray optical elements from the vacuum vessel. Moreover, it is necessary to perform a strict alignment when re-installing the soft x-ray optical elements in the vacuum vessel after cleaning. Such a procedure requires considerable time, and is therefore undesirable. As a result, a technique is needed that makes it possible for removing substances adhering to the surfaces of soft x-ray optical elements with the elements left in place without removing the elements from the vacuum vessel.
The problem with an LPX, as described in the foregoing description, will be described now in more detail. As mentioned in the foregoing description, in LPXs, ions, atoms and small fragments of the target material are emitted from the plasma or the vicinity of the plasma in addition to x-rays. Such stray matter may deposit or adhere to optical elements disposed in the vicinity of the plasma. For example, such stray matter may deposit on windows used to introduce the laser light into the vacuum vessel, lenses used to focus the laser light in cases where focusing elements are disposed inside the vacuum vessel, mirrors used to reflect the x-rays radiated from the plasma, and filters used to cut visible light but pass x-rays radiated from the plasma. As a result, this stray matter causes a drop in the reflectivity and transmissivity of the optical elements. Accordingly, removing such stray particles as well as reducing the amounts of such stray matter are important problems in the utilization of LPXs.
Methods have been used to remove and reduce such stray matter. For example, substances that are gases at ordinary temperatures such as nitrogen, carbon dioxide, krypton or xenon are used as the target material. These gases or clusters jet from the nozzle and are irradiated with laser light. Since such target materials are gases at room temperatures, these materials are not deposited on the surfaces of optical elements, and therefore cause no deterioration in the performance of the optical elements. However, this method using these gases also has problems, which will be described next.
In an LPX in which a substance that is a gas at room temperature is used as a target material, the gas that is caused to jet from the nozzle spreads throughout the interior of the vacuum vessel due to free expansion. As a result, the density of the gas decreases abruptly as the distance from the nozzle increases. However, in order to increase the quality of x-rays emitted from the plasma, it is necessary to generate plasma in the vicinity of the nozzle at a distance ranging from several tenths of a millimeter to several millimeters where the gas density is large. Moreover, when plasma is generated in the vicinity of the nozzle, the high-velocity atoms, ions, and electrons emitted from the plasma collide with the nozzle and members located in the vicinity of the nozzle and abrade these elements. As a result, these stray particles, the atoms or small fragments of the nozzle, or members located in the vicinity of the nozzle scatter into the surrounding areas and adhere on the optical elements disposed in the vicinity of the plasma. As a result, the performance of the optical elements drops.
Thus, in cases where a gas or cluster-form target material is used, the plasma must be positioned close to the nozzle to increase the efficiency of energy conversion from laser light to x-rays. However, this causes an increase in the quantity of stray particles scattered from the nozzle and members located in the vicinity of the nozzle. As a result, in LPXs using this gas method, it is difficult to achieve both an improvement in the efficiency of energy conversion from laser light to x-rays and a reduction in the quantity of stray particles.
Furthermore, a slight back-flow of oil may result from the rotary pump into the vacuum system. As a result, this oil deposits on the x-ray optical elements, and causes a gradual drop in the performance of reflectivity, transmissivity, and diffraction efficiency of the x-ray optical elements. In such cases, the only available methods have been methods in which the x-ray generating apparatus is disassembled, and the optical elements are replaced with new elements, or methods in which the optical elements are removed from the apparatus, cleaned and then returned to the apparatus.
Accordingly, the present invention is directed to an x-ray apparatus, and a x-ray generating apparatus for use as an x-ray source in the x-ray apparatus that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an x-ray apparatus in which substances adhering to the surfaces of soft x-ray optical elements can be removed without removing the elements from the vacuum vessel.
Another object of the present invention is to provide an LPX in which long-term continuous operation is possible, foreign matter deposited on or adhering to optical elements can be removed, and long-term operation is possible without replacing or cleaning the optical elements even in cases where portions of the nozzle or members located in the vicinity of the nozzle have been abraded away by stray particles such as atoms, ions or electrons emitted from the plasma.
Additional features and advantages of the invention will be set forth in the description which follows, and will be apparent from the description, or may be learned by practice of the invention. The objects and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages in accordance with the purpose of the invention, as embodied and broadly described herein, an x-ray apparatus includes an x-ray source; an x-ray optical system, wherein the x-ray optical system extracts x-rays of a specific wavelength from the x-ray source and converts the x-rays into a beam; and an irradiation source that causes radiation to be incident on surfaces of optical elements contained in the x-ray apparatus.
In another aspect, the present invention provides an x-ray generating apparatus for use as an x-ray source in an x-ray apparatus that includes a vacuum vessel; a nozzle having a tip end and a body for jetting a gas to be used as a target substance inside the vacuum vessel; a pulsed laser light source for irradiating the target substance, wherein the irradiation causes plasma to be generated from the target substance, and wherein the plasma radiates x-rays; an irradiation source that causes radiation to be incident on surfaces of optical elements contained in the x-ray generating apparatus, wherein the irradiation source is an ultraviolet light source, a vacuum ultraviolet light source, or a light source of a wavelength shorter than the wavelengths of ultraviolet light or vacuum ultraviolet light; and a gas introduction mechanism that introduces either oxygen or ozone, both oxygen and ozone, or a gas containing at least oxygen or ozone into the vacuum vessel or areas surrounding the optical elements that are being irradiated by the irradiation source.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.